refactor(core): 1062 唯一键冲突判定收口至 core/db_errors

六个 service 各自复制的 _is_duplicate_entry（口径已漂移：rating/product
用 _ERR_DUP_ENTRY 常量、其余四处写字面量 1062）收敛为共享函数；
is_duplicate_entry 语义与文案注释各处保留，非 1062 完整性错误仍原样
上抛由全局处理器收敛。

Co-Authored-By: Claude Code <noreply@anthropic.com>

backend/app/services/shipment_service.py

@@ -35,6 +35,7 @@ from app.core.constants import (
     MODULE_SHIPMENT,
     NO_PREFIX_SHIPMENT,
 )
+from app.core.db_errors import is_duplicate_entry
 from app.core.exceptions import BadRequestError, NotFoundError
 from app.core.response import PageResult
 from app.models import Business, Shipment, ShipmentItem, User
@@ -144,8 +145,7 @@ def _ensure_business_writable(session: Session, business_id: int, action: str) -
 
 def _is_duplicate_entry(exc: IntegrityError) -> bool:
     """识别 MySQL 唯一键冲突（1062），与 business/receipt_service 同口径。"""
-    args = getattr(exc.orig, "args", ())
-    return bool(args) and args[0] == 1062
+    return is_duplicate_entry(exc)
 
 
 def list_shipments(session: Session, query: ShipmentQuery) -> PageResult[ShipmentOut]: